It is known to add or embed codes in broadcast audio and/or video and/or vertical blanking interval signals. For example, codes may be embedded in television and/or radio broadcasts and/or in pre-recorded audio or video content. In the field of audience metering, codes can be added to audio and/or video signals for the purpose of, for example, identifying programs and/or the distributor(s) that are broadcasting the programs, identifying commercials and promotional announcements, and the like.
Codes that are added to audio signals can be reproduced in the audio signal output by a speaker. Accordingly, these arrangements offer the possibility of non-intrusively intercepting and decoding the codes with equipment that uses microphonic inputs. For example, these systems enable measuring broadcast audiences by the use of portable metering equipment carried by panelists.
Audio codes are inserted at low intensities to prevent the codes from distracting a listener of program audio and, therefore, such codes can be vulnerable to various signal processing operations. Consequently, these approaches to encoding a broadcast audio signal may not be compatible with current and proposed digital audio standards, particularly those employing signal compression methods that can reduce the dynamic range of a signal. Dynamic range reduction processing of an audio signal may delete or damage an audio code inserted in the audio signal. In this regard, it is particularly important for an audio code to survive compression and subsequent de-compression carried out by, for example, the Dolby™ Digital Audio Code Number 3 (AC-3) Surround Sound algorithm or by one of the algorithms recommended in the Moving Picture Experts Group (MPEG) standards (e.g., MPEG-1, MPEG-2, MPEG-4, and the like).
Systems and methods for adding an inaudible code to an audio signal and subsequently retrieving that code in a manner that is compatible with current and proposed digital audio standards are known. In one such system, an encoder is arranged to add a binary code bit to a signal block by selecting, within the signal block, (i) a reference frequency within the predetermined signal bandwidth, (ii) a first code frequency having a first predetermined offset from the reference frequency, and (iii) a second code frequency having a second predetermined offset from the reference frequency. The spectral amplitude of the signal at the first code frequency is increased so as to render the spectral amplitude at the first code frequency a maximum in its neighborhood of frequencies and is decreased at the second code frequency so as to render the spectral amplitude at the second code frequency a minimum in its neighborhood of frequencies. A decoder can be arranged to decode the binary bit.
However, the extraction of audio codes from audio signals in such digital-audio-compatible systems requires considerable processing power, because complicated mathematical operations are used for code extraction. If several channels exist, it may be necessary to extract the audio codes from the numerous channels simultaneously. For example, DOLBY™ Digital AC-3 Surround Sound delivers six separate (discrete) channels of sound. AC-3 includes left, center, and right channels across the front of the room and separate (discrete) left and right surround sound channels. The sixth channel is a Low Frequency Effects Channel that is typically coupled to a sub-woofer or the like. With six separate channels, considerable processing power may be required to extract audio codes from each of the channels simultaneously.